Examples of conventionally known optical discs to/from which information is recorded/reproduced include CDs, DVDs, etc. When recording/reproducing information to/from a conventional optical disc, an incident angle of light incident on a surface of the optical disc does not have a great value.
FIG. 7 illustrates an exemplary configuration including a conventional optical disc 508 and an optical head 513 for recording/reproducing information to/from the optical disc 508. The optical head 513 includes a radiation source 501, a beam splitter 502, a collimating lens 503, an objective lens 506, a hologram 511, and a photodetector 512.
When reproducing information recorded on the optical disc 508, a light beam emitted by the radiation source 501 is transmitted through the beam splitter 502 and rendered into a parallel beam by the collimating lens 503. The parallel beam enters the objective lens 506 and is converged on an information recording layer 711 of the optical disc 508.
Generally, a semiconductor laser is used as the radiation source 501. An emission wavelength of the semiconductor laser is in the range of about 780 nm to about 810 nm when the optical disc 508 is a CD and in the range of about 630 nm to about 670 nm when the optical disc 508 is a DVD.
The light beam is reflected by the information recording layer 711 and transmitted, again, through the objective lens 506 and the collimating lens 503. Then, the light beam is reflected by the beam splitter 502 to enter the photodetector 512. In the photodetector 512, an information signal representing information recorded on the optical disc 508 and a servo signal for tracking are retrieved from the light beam.
FIG. 8 illustrates a light beam 701 incident on the optical disc 508. The light beam 701 includes light 708 vertically incident (i.e., an incident angle is 0°) on a surface of the optical disc 508 and lights 707 and 709 incident on the optical disc 508 at an incident angle α. The light beam 701 is converged on the information recording layer 711 to form a light spot 712.
In recent years, for the purpose of recording multimedia data including large-sized dynamic image data or the like, there has been a demand to increase the density of information to be recorded to the optical disc 508. One method for increasing the density of information to be recorded to the optical disc 508 is to reduce the size of the light spot 712 to be formed on the information recording layer 711.
The diameter φ of the light spot 712 is represented by the following expression (1):φ=k·λ/(NA)  (1),where is λ wavelength of the light beam 701 (i.e., wavelengths of lights 707, 708, and 709), k is a constant, and NA is a numerical aperture of the objective lens 506 (FIG. 7). Constant k is determined according to light distribution at an entrance pupil. Constant k is small when the light distribution at the entrance pupil is uniform, and constant k is large when the light distribution at the entrance pupil is not uniform (e.g., light around the periphery of the entrance pupil is weaker than that in a central portion of the entrance pupil).
As can be appreciated from expression (1), in order to reduce the size of the light spot 712, it is necessary to: (1) increase the numerical aperture NA of the objective lens; (2) uniformly distribute light over the entrance pupil to reduce constant k; or (3) reduce wavelength λ of the light beam.
The wavelength λ of the light beam 701 is determined based on an emission wavelength of the radiation source (e.g., a semiconductor laser) 501 (shown in FIG. 7). In recent years, a semiconductor laser having a short emission wavelength, such as a blue semiconductor laser, is used to reduce the size of a light spot based on the above-mentioned method (3). However, in the case of a conventional optical disc, when the numerical aperture NA of the objective lens is increased, the maximum possible incident angle of light incident on a surface of the optical disc becomes large, and thus a reflectance for the light becomes large, so that light distribution at the entrance pupil cannot be uniform.
FIG. 9 shows a relationship between an incident angle of light incident on the conventional optical disc 508 (FIG. 8) and a reflectance for the light reflected from a surface of the optical disc. In FIG. 9, dependence of a reflectance on an incident angle is shown with respect to each of S-polarized light (indicated by S in the figure) and P-polarized light (indicated by P in the figure). Note that the P-polarized light refers to polarized light having an electric vector parallel to a cross section of incidence (i.e., a plane including a normal line of a plane on which light is incident and the incident light), and the S-polarized light refers to polarized light having an electric vector perpendicular to the cross section of incidence.
From FIG. 9, even when the S- and P-polarized lights have the same incident angle, respective reflectances for the S- and P-polarized lights are different from each other. Light incident on the objective lens 506 (FIG. 7) is generally circularly polarized light, and thus an average reflectance for the light beam 701 shown in FIG. 8 from the surface of the optical disc 508 corresponds to an intermediate value (shown as (S+P)/2 in the figure) between the reflectances for the S- and P-polarized lights. Hereinafter, a reflectance for light refers to the intermediate value ((S+P)/2) between the reflectances for the S- and P-polarized lights unless otherwise indicated.
FIG. 9 shows that reflectances for the P- and S-polarized lights are approximately 0% and 20%, respectively, when the incident angle is in the vicinity of 58°. Note that the maximum possible incident angle and the numerical aperture NA of the objective lens 506 are related to each other such that the maximum possible incident angle is increased along with an increase of the numerical aperture NA of the objective lens 506. The maximum possible incident angle α=58.2° corresponds to the numerical aperture NA=0.85 of the objective lens 506.
It is appreciated that in the case of the conventional optical disc 508, when the numerical aperture NA of the objective lens 506 (FIG. 7) is increased (e.g., when the numerical aperture NA is increased to such an extent that the incident angle is greater than 45°), the average reflectance indicated as (S+P)/2 in the figure is abruptly increased. A large reflectance means that light transmitted through the optical disc 508 and reaching the light spot 712 is weak. Accordingly, in the case of the conventional optical disc 508, when the numerical aperture NA of the objective lens 506 (FIG. 7) is increased, light around the periphery of the entrance pupil (e.g., light having entered the optical disc 508 at the maximum possible incident angle α and reached the light spot 712) becomes weak. Weakening of light around the periphery of the entrance pupil is equivalent to a relative increase of a value of constant k in the above expression (1). Accordingly, an effect of reducing the size of the light spot 712 cannot be achieved, regardless of the increased numerical aperture NA.
As described above, with a conventional optical disc, the light spot cannot be sufficiently small, and thus it is not possible to increase a recording density of information.
The present invention has been made in view of the above problem and an objective thereof is to provide an information recording medium capable of increasing the recording density of information. Another objective of the present invention is to provide an information recording/reproducing apparatus using the same information recording medium.